Hepatocellular carcinoma has been causally related to chronic infection by hepatitis B virus (HBV). Prevention of new HBV infection can now be achieved by vaccination. However, no antiviral therapy is available to combat chronic HBV infection, once infection occurs. Thus, the large number of HBV carriers are at a very high risk to hepatocellular carcinoma. Our laboratory has been studying hepatitis delta virus (HDV), which infects only HBV carriers. HDV is a defective RNA virus, but can inhibit HBV DNA replication. Thus, the interactin between HDV and HBV represents a unique case of RNA-DNA interaction. Therefore, the understanding of the molecular biology of HDV and the interaction between HDV and HBV may provide us with a new perspective to the control of HBV replication and reduction of incidence of hepatocellular carcinoma. HDV infection frequently causes severe fulminant hepatitis. This project proposes to use hepatoma cell lines expressing various HBV components and nonhepatic cell lines to study the molecular biology of HDV replication and interaction between HDV and HBV. Since HDV contains a circular single-stranded RNA genome, which has no parallel among animal viruses, this project promises to provide important fundamental information of molecular biology. The following specific projects will be carried out: (1) Perform RNA sequencing of HDV strains from different geographical regions to determine the possible heterogeneity of the virus. (2) Transfect a dimer HDV cDNA into different cell lines, including hepatoma and nonhepatic cell lines to determine the mechanisms of HDV RNA replication and of HBV-HDV interaction. We will examine the synthesis of viral proteins and RNA, and possible production of virus particles. (3) Transfect dimer RNAs into different cell lines to study RNA processing. (4) Express different open reading frames to study their possible gene products and function. This project is expected to yield information on the mechanism of synthesis of a novel class of RNA in mammalian cells, and provide a possible mechanism for inhibition of HBV, which is the major cause of hepatocellualar carcinoma.